Data type conversion improvement

[dai-chen]

Is your feature request related to a problem? Please describe.
Currently the type check is very strict and thus not very user friendly. We need to support implicit casting and make it consistent with explicit casting. For example: if A can be converted to B implicitly, this should be done by explicit CAST function too.

1. Comparing date, timestamp and string lead to failures and Date formatted as timestamp opendistro-for-elasticsearch/sql#1056: Comparison between date and string
2. WHERE boolField = 'true', doesn't autocast (changed behavior) opendistro-for-elasticsearch/sql#1061: Comparison between bool and string
3. Regression?date_filed>1591459200000 does not works in latest version opendistro-for-elasticsearch/sql#510: Comparison between date and long?

Cast function in PPL: opendistro-for-elasticsearch/sql#924

More SQL types supported:

1. DECIMAL
2. SIGNED INT
3. CHAR
4. etc

Describe the solution you'd like
Relax the type check by well defined conversion rules.

Describe alternatives you've considered
Follow the strict data type grammar, such as TIMESTAMP(...).

[dai-chen]

More SQL type support will be worked on as needed in future. Closing for now.